(1) Field of the Invention
This invention relates to the treatment of water, particularly water intended for use in fish farming plants. More explicitly the invention relates to a method for water treatment comprising aeration of the water to be treated and causing the water to circulate to a chamber, into an adjacent basin of considerably larger volume than that of the chamber and back to the chamber, the aeration producing bubbles acting to form a foam containing dissolved impurities at the surface of the water in the chamber, and removing the foam from the chamber as it is formed.
(2) State of the Prior Art
Such a method is disclosed in Norwegian patent No. 58 996. According to this patent the circulation of the water is provided by an external water pump in which the air for aeration of the water is added before the water is ejected into an upper portion of the chamber just below the foam formed by the air bubbles.
It should be generally noted that water treatment based on air bubbles rising in water takes place by two strictly distinguished phenomena or mechanisms, termed flotation and physical adsorption, respectively, which, dependent on the size of the bubbles, take care of different forms of impurities in the water. Flotation is used to remove particulate organic and inorganic material suspended in the water, while physical adsorption extracts dissolved organic and inorganic substances in the water.
Flotation utilizes the ability of the suspended particles to rise to the surface of a liquid. For this to happen the density of the particles must be lower than that of the liquid or they must be able to be lifted to the surface by gas bubbles enclosing or adhering to the material, which settles as a slurry deposit on top of the water, from where it can be removed. However, the water surface must be relatively calm, as otherwise the slurry will disintegrate.
In physical adsorption, polarized compounds dissolved in the water associate with the air bubbles, which bring them to the surface where they are collected in a voluminous bubble foam that is easily removed.
The flotation process uses air bubbles ranging from about 0.03 mm up to about 0.12 mm in size, while the physical adsorption process is most effective with bubbles of a size around 0.8 mm.
Normally the air bubbles for flotation are produced by dissolving air in water that is put under a pressure of several atmospheres and then releasing the pressure to form slowly rising gas bubbles of the above size due to supersaturation. As an example of this flotation technique, refer to SE patent No. 169 561 which relies on the use of pumps, valves, water pipes and a pressure chamber. Also, in connection with fish farming, this conventional flotation method will poison the water if air is used as gas medium, since the water will become supersaturated with nitrogen gas.
The bubbles used in physical adsorption are normally produced by an air diffusor, i.e. a tube, hose or the like, having air permeable walls and positioned at the bottom of a column of polluted water. For a more detailed description of physical adsorption, refer to the book "Seawater Aquariums" by Stephen Spotte, John Wiley & Sons, 1979, p. 208-217.
A closer study of the description of the above Norwegian patent No. 58 996 will make it clear that, although it is not directly mentioned therein, the method of the patent obviously relies on physical adsorption of the dissolved substances rather than on flotation of particulates. It is mentioned that the bubbles in the surface foam easily burst and the foam collapses, which is typical for a foam formed through physical adsorption with large bubbles. Further, the tiny air bubbles (which could cause flotation) are considered to be a problem, and therefore should be removed by means of a sponge filter in the chamber while particulate material is removed by an external mechanical filter. Furthermore, the fact that the water is ejected into the chamber close to the water surface will cause turbulence, destroying any flotation effect that might emerge. Consequently the patent teaches that the foam has to be rapidly removed to prevent the impurities from returning into the water. (Physical adsorption, in contrast with chemical adsorption is a reversible process using weak bonds, such as those formed by van der Waals forces). Therefore, this prior water treatment method which, according to the patent, was proposed more than 50 years ago, does not produce water of a quality satisfying the demands of modern, full scale fish breeding plants. Because the retention time of the particulate material in the system is too long, and the material also accumulates within the fish basins. Furthermore, ammonia will also accumulate in the system.